Method of making piezoelectric crystal units

ABSTRACT

Crystal plate mounting means integral with malleable terminals of a piezoelectric crystal unit accommodate differently dimensioned crystal plates and permit spacing of the terminals for registry with circuit board perforations without stressloading such plates. Other means integral with the terminals isolate the crystal plate from stress-loading when the free ends of the terminals are stressed. The illustrated mounting means comprise bifurcations formed in one end of each terminal and the illustrated other means comprise a paddle section of each terminal embedded in a resilient organic adhesive securing and hermetically sealing each terminal to an eyelet. The malleable terminals are solderable and readily deformable to secure registry between the free ends of the terminals and perforations in printed circuit boards. The organic sealant is compatible with the malleable terminal material and withstands stresses induced therein when the terminals are stressed and when the eyelet is cold welded to an envelope. The disclosed method includes the steps of securing a pair of terminals to an eyelet, adapting the ends of the terminals to support a crystal plate without stressloading such crystal plate, securing a crystal plate in a stress free condition to the terminals, and cold welding the eyelet to an envelope by a cold weld processing step to avoid mass-loading the crystal plate.

United States Patent Scott, Jr. et al.

[54] METHOD OF MAKING PIEZOELECTRIC CRYSTAL UNITS [72] Inventors: Kelley E. Scott, Jr., Plano; Daryl M.

Kemper, Sandwich, both of lll.; Lloyd E. Grove, Geneva; Ronald J- Kiess, Decatur,

both of Ind.

[73] Assignee: CTS Corporation, Elkhart, Ind. [22] Filed: June 6, 1969 [21] Appl.No.: 830,956

[52] U.S.Cl. ..29/25.35, 29/630 G,310/8.9, 3l0/9.1, 310/94 [51] lnt.Cl. ..B0lj 17/00, H04r17/00 [58] FleldotSearch ..29/25.35,630G;310/9.1,9.4, 310/8.9 [56] References Cited UNITED STATES PATENTS 2,784,326 3/1957 Purdue ..310/9.4 2,785,321 3/1957 lmler ..310/9.1 2,857,532 10/1958 Ziegler ..310/8.9 3,022,431 2/1962 McKnight.... ..3l0/9.4 3,054,915 9/1962 l-louck ..310/9.1 3,340,410 9/1967 Sanford ..3lO/9.1

Primary Exa r iin e .lohn F. Campbell W 15] 3,656,217 [451' Apr. 18,1972

[57] ABSTRACT Crystal plate mounting means integral with malleable terminals of a piezoelectric crystal unit accommodate differently dimensioned crystal plates and permit spacing of the terminals for registry with circuit board perforations without stress-loading such plates. Other means integral with the terminals isolate the crystal plate from stress-loading when the free ends of the terminals are stressed. The illustrated mounting means comprise bifurcations formed in one end of each tenninal and the illustrated other means comprise a paddle section of each terminal embedded in a resilient organic adhesive securing and hermetically sealing each terminal to an eyelet. The malleable terminals are solderable and readily deformable to secure registry between the free ends of the terminals and perforations in printed circuit boards. The organic sealant is compatible with the malleable terminal material and withstands stresses induced therein when the terminals are stressed and when the eyelet is cold welded to an envelope. The disclosed method includes the steps of securing a pair of terminals to an eyelet, adapting the ends of the terminals to support a crystal plate without stress-loading such crystal plate, securing a crystal plate in a stress free condition to the terminals, and cold welding the eyelet to an envelope by a cold weld processing step to avoid mass-loading the crystal plate.

6 Claims, 7 Drawing Figures PATENTEDAPR 18 I972 SHEETEUFZ TEMPERATURE E AIIAEQQ 0V m:= o QEBMEw FIGURE-5 FIGURE-6 m Tm T W OMOE W. N mSKG m E 0. vi T VEM T WY DGUA LD LYYA LRON EALO KDLR M METHOD OF MAKING PIEZOELECTRIC CRYSTAL UNITS This invention relates to piezoelectric crystal units and, more particularly, to an improved construction of such units and to a method of making the same.

The satisfactory use of crystal units in frequency control circuits is predicated on the long term accuracy and frequency stability of such units. It will be appreciated that even when it is perrnissable for a crystal unit to exhibit a change in operating frequency of 3 X parts per day, it is necessary to prevent mass-loading of the crystal plate, i.e., alteration of the resonant characteristics of a crystal plate because of the deposition of contaminants such as water vapor, dust, or other organic or inorganic materials on one or more surfaces of the crystal plate. In addition to avoiding mass-loading, it is desirable to avoid stress-loading of the crystal plate, i.e., alteration of the resonant characteristics of a crystal plate because of mechanical stresses applied to the crystal plate by the means that are used to mount the crystal plate in an enclosure or envelope.

Because of the importance of operating a crystal plate in a contaminant-free environment, elaborate and expensive techniques have been used in the past to hermetically seal one or more crystal plates within a contaminant-free enclosure. This expedient has also been utilized in order to avoid the frequency shift problems that can arise due to the occurrence of chemical reactions involving the electrode material deposited on the crystal plate. Prior art sealing techniques have comprised the steps of hermetically sealing a pair of terminals in an eyelet to form a header, mounting a crystal plate on the header, and hermetically sealing the header to a metal or glass envelope with the crystal plate disposed within such envelope.

Heretofore, headers have been characterized either as matched glass" or compression glass headers. In a matched glass header, a single material is used to fabricate the terminals and eyelets; and a vitreous material, having the same thermal coefficient of expansion as the eyelet and terminals, seals the terminals to the eyelet. Normally, the eyelet and terminals are made of Kovar, and the vitreous material is a glass that has been selected to have a thermal coefficient of expansion substantially the same as that of Kovar. The sealing process is accomplished by placing molten or liquid glass in the eyelet around the terminals and then cooling the glass to cause it to shrink and tightly grip the terminals, and become vitreous. Thereafter, the similar coefficients of thermal expansion of the Kovar and vitreous glass ensure that the seal between the terminals and eyelet will be maintained. Compression glass headers are made by generally following the same process steps used to make matched glass headers. However, the vitreous glass is selected to have a coefficient of thermal expansion greater than the terminal material, and the eyelet material differs from the terminal material and is selected to have a coefficient of thermal expansion greater than the vitreous glass. In the processing of these headers, the glass continues to shrink around the terminals after it becomes vitreous whereas the eyelet shrinks around the vitreous glass as the header is cooled.

In both matched glass and compression glass headers, the integrity of the terminal to eyelet seal depends upon the attainment of a good seal between the terminal and glass. In turn, this seal is dependent at least partly upon the presence of a tenacious oxide coating on the portion of the terminal embedded within the glass. As will be understood by those skilled in the art, the requirement for a tenacious oxide coating on the terminal and the necessary exposure of the terminal to a molten glass inherently precludes the use of many potential terminal materials because of the extreme corrosiveness of molten glass and the ease with which many metallic oxides will dissolve in molten glass. This is true in fact, even when glasses having a melting point of as low as 450 to 700 C. are used as the vitreous sealant. More specifically, the corrosiveness of molten glass has generally precluded the use of terminal materials such as copper or tin coated copper. Accordingly, it

would be desirable to provide a new and improved hermetically sealed crystal unit wherein a sealant and fabricating process is used that would permit the use of terminals fabricated from copper, tin coated copper, or other easily soldered electrically conductive malleable materials.

The integrity of conventional terminal to eyelet seals also depends, among other things, on the change in coefficient of thermal expansion that is exhibited as a molten glass is cooled from a liquid and supercooled liquid state to a vitreous state. As is well known to persons skilled in the glass art, any given glass is characterized by a relatively constant coefiicient of thermal expansion while the glass is vitreous and below the transformation range temperatures of such glass. At temperatures above the transformation range, i.e., normally above about 600 C., the coefficient of thermal expansion of a given glass increases by a factor of two or three. In practice, this means that when a vitreous glass with a coefiicient of thermal expansion equal to Kovar is heated to a liquid state, i.e., to a temperature of l,200 to l,500 C., and then allowed to cool, the glass will shrink or contract two to three times as much, per unit measure, as Kovar until a fictive temperature of the glass is reached. Then, the glass becomes vitreous and in theory contracts at the same rate as Kovar. Since the increased rate of contraction takes place over a range of 300 to 600 Centigrade after the glass has macroscopically become a solid, it will be appreciated that the increased contraction of the glass results in a compressive type seal even between such glass and Kovar and this compression inevitably creates stresses in the vitreous sealant that in turn make such sealant relatively susceptible to crazing or cracking. In addition, the observable fictive temperature of a glass varies as a function of the cooling rate of the glass and the actual compressive forces exerted by a vitreous glass cannot be precisely predicted unless the cooling rate of the glass is precisely controlled.

The relative ease with which vitreous materials may be cracked or broken during handling places limitations on many of the process steps that may be practiced during the manufacture of crystal units. For example, only a minimum amount of stress may be applied to a header when securing together the header and envelope in order to avoid damaging the vitreous sealant and thereby destroying the hermetic seal between the terminals and eyelet. Accordingly, it would be desirable to provide a new and improved hermetically sealed crystal unit wherein the header may be subjected to relatively great stresses during a manufacturing process without damaging the hermetic seal. The known hermetic sealants are also susceptible to damage while being installed in electrical equipment making use of standard printed circuit boards. For example, slight deviations in the relative positions of the crystal unit terminals and circuit board perforations can prevent registry of the terminals with such apertures. In such cases, the relatively stiff terminals must be bent or otherwise deformed with the concomitant risk of stressing the vitreous sealant and destroying the hermetic seal.

It is desirable to precisely locate crystal unit terminals so that they will register with apertures in printed circuit boards, and it is also desirable to very precisely locate the terminals so that they can support the crystal plate. In some applications, the required spacing between terminals for proper registry with a printed circuit board prevents optimum terminal spacing from the viewpoint of supporting the crystal plate. In these situations, it is necessary to space the terminals for registry with the circuit board and attach to the terminals separate crystal plate mounting means that resiliently grip the crystal plate and facilitate the completion of a solder connection between the mounting means and electrodes on the crystal plate. In addition to increasing the cost of a crystal unit, this arrangement is objectionable because the mounting means apply a compressive force and stress-load the crystal plate. It therefore would be desirable to provide improved crystal plate mounting means that accommodate variously sized crystal plates without stress-loading such crystal plates.

When separate crystal plate mounting means are spot welded or otherwise secured to the ends of terminals it is necessary to specifically orient the terminals within the eyelet so as to maintain proper orientation of the mounting means relative to the eyelet while a hermetic sealant is applied to secure the terminal to the eyelet. When separate mounting means have been secured to the terminals after the hermetic seal has been completed, it has still been necessary to maintain a precise orientation of the mounting means relative to the eyelet while securing the mounting means to the terminals. Accordingly, it would be desirable to provide an improved mounting means and method of manufacturing a crystal unit that eliminates the necessity of maintaining the critical orientation of mounting means relative to an eyelet while making a hermetic seal between the terminals and such eyelet or while securing the mounting means to the terminals.

In contemporary crystal units, metal eyelets are soldered or brazed to metal envelopes. The use of these high temperature techniques increases the risk of mass-loading as the result of vapors forming a deposit on a crystal plate within an envelope. The fact that metallic vapors may form a deposit on a crystal plate and thereby cause a change in frequency is well known and described in the Klingspom U.S. Pat. No. 3,028,262, dated Apr. 3, 1962, and entitled Method For The Frequency Tuning Of Piezoelectric Oscllators. It will thus be appreciated that any means used for hermetically sealing an eyelet to a metallic envelope that involves the use of heat or molten materials can potentially result in mass-loading a crystal plate within the envelope. Because of these problems, attempts have been made to secure metal eyelets to a metal envelope by means of apparatus that do not involve the use of heat or molten materials. These attempts have involved the use of apparatus and equipment of the type described in Sowter U.S. Pat. No. 2,522,408 entitled Cold Pressure Welding." In practice, however, these efforts have not been completely satisfactory because the stresses created in the eyelet during the cold welding process can easily damage the vitreous sealant material in the eyelet. Accordingly, it would be desirable to provide an improved crystal unit wherein a metal eyelet is cold welded to a metal envelope without damaging the hermetic seal between a pair of terminals and the eyelet.

Accordingly, it is an object of the present invention to provide a new and improved piezoelectric crystal unit. Another object of the present invention is to provide a new and improved method of making piezoelectric crystal units. A further object of the present invention is to provide a new and improved hermetically sealed crystal unit wherein the hermetic sealant accommodates and permits the use of terminals made from a readily solderable malleable material. An additional object of the present invention is to provide a new and improved crystal unit that is capable of withstanding manufacturing and handling stresses without damage to hermetic seals associated therewith. Yet another object of the present invention is to provide a new and improved means for mounting a crystal plate that does not stress-load such plate. Yet a further object of the present invention is to provide a new and improved means for mounting a crystal plate that dispenses with the need for separate structural elements attached to the terminals of the crystal unit and that will accommodate variously dimensioned crystal plates. Yet an additional object of the present invention is to provide a new and improved crystal unit wherein the terminals thereof may be deformed and stressed during assembly and subsequent handling without stress-loading the crystal plate and without damaging a hermetic seal between the terminals and eyelet during such deformation. Still another object of the present invention is to provide a new and improved crystal unit incorporating a sealant means that is compatible with malleable and readily solderable terminals. Still a further object of the present invention is to provide an improved crystal unit that facilitates the assembly of a pair of terminals and an eyelet without regard to the orientation of crystal plate mounting means relative to the eyelet. A more specific object of the present invention is to provide a new and improved method of manufacturing a crystal unit wherein a crystal plate is supported directly by a pair of terminals. A still more specific object of the present invention is to provide a new and improved crystal unit wherein an eyelet is hermetically sealed to an envelope without massloading a crystal plate within the envelope. An even more specific object of the present invention is to provide a new and improved crystal unit having a resilient hermetic sealant capable of withstanding stresses induced therein. These and other objects and advantages of the present invention will become apparent as the following description proceeds, and the features of novelty characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

The present invention is concerned with a piezoelectric crystal unit that preferably is hermetically sealed. Crystal units embodying the present invention include an envelope, a header comprising an eyelet and terminals, and a crystal plate supported by mounting means integral with the terminals. The mounting means accommodate diflerently dimensioned crystal plates and permit spacing of the terminals for registry with a circuit board without stress-loading such plates. Other means, integral with the terminals, isolate the crystal plates from stress-loading when the free ends of the terminals are bent or otherwise deformed. In the illustrated embodiment, the mounting means comprise bifurcations that are cut, abraded, or otherwise formed in one end of each terminal. Preferably, the mounting means are formed after the terminals have been assembled with an eyelet in order to avoid maintaining precise orientation of the mounting means relative to each other during assembly of the terminals and eyelet. The exemplified other means integral with the terminals for isolating the crystal plate from stress-loading comprise a paddle section of each terminal that is embedded in the means used to secure the terminals to the eyelet. Preferably, the terminals are made from a malleable material such as aluminum, copper, or tin-coated copper. Other aspects of the invention are concerned with using a cold-welding process to secure together the header and envelope and with using a resilient non-vitreous material such as an organic adhesive material to secure and hermetically seal the terminals to the eyelet. The non-vitreous sealant is compatible with malleable terminal materials and maintains the integrity of a hermetic seal when the header and envelope are cold-welded.

For a better understanding of the present invention, reference may be had to the accompanying drawings wherein the same reference numerals have been applied to like parts and wherein:

FIG. 1 is an isometric view of a piezoelectric crystal unit embodying features of the present invention;

FIG. 2 is an exploded isometric view of the crystal unit illustrated in FIG. 1;

FIG. 3 is a cross-sectional view taken along the lines Ill-III in FIG. 1;

FIG. 4 is a cross-sectional view taken along the lines lVIV in FIG. 3, assuming that the crystal unit in FIG. 3 is shown in full;

FIG. 5 is a graph showing the relationship between the specific volume and temperature of a vitreous material and a non-vitreous material;

FIG. 6 is a view similar to FIG. 4 illustrating another embodiment of the invention; and

FIG. 7 is a view similar to FIGS. 4 and 6 illustrating still another embodiment of the invention.

Referring now to the drawings, and more particularly to FIG. 1, a crystal unit ill embodying the present invention comprises an envelope 11, a pair of terminals 12, 13, and an eyelet 14 comprising an apertured base, a continuous sidewall connected to the base, and a flange extending from a peripheral edge of the sidewall. Means for supporting the terminals on the eyelet include a hermetic sealant that comprises a resilient organic adhesive 16 which maintains a hermetic seal between the terminals and eyelet. As best illustrated in FIGS. 3 and 4, a crystal plate 17 is supported by mounting means that form an integral part of the terminals 12, 13. Deposits of a conductive adhesive material such as solder or epoxy mechanically secure and electrically connect the mounting means with a pair of conventional crystal plate electrodes 19, 21. As best illustrated in FIGS. 2 and 3, the mounting means comprise bifurcated portions 22, 23 of the terminals and when the crystal plate 17 is positioned on such bifurcated portions, deposits of conductive epoxy 24 fixedly secure the crystal plate 17 to the bifurcations 22a, 22b, 23a, and 23b. The bifurcations on each terminal preferably are spaced apart a distance slightly greater than the thickness of the thickest crystal plate expected to be supported by the terminals l2, 13 so that a crystal plate may be readily slipped into place between the bifurcations on each terminal without being stress-loaded. In FIG. 2, the terminal 12 has been rotated slightly to better illustrate this spacing. When being mounted on the terminals 12, 13, the crystal plate 17 is positioned without constraint between the bifurcations, and conductive epoxy 24 is deposited to secure the crystal plate to the bifurcations. After the epoxy 24 has cured and become relatively rigid, the crystal plate 17 is firmly supported by the terminals 12, 13 and yet remains in a relaxed or unstressed condition. Although it is normally preferred to form each of the mounting means with two bifurcations as illustrated because of the ease with which the crystal plate 17 is assembled therewith, the mounting means may be embodied in other forms. For example, one of the bifurcations may be removed from each terminal so that each mounting means comprises a single bifurcation disposed against a face of the crystal plate and a ledge or shoulder at the base of such single bifurcation for supporting the bottom edge of the crystal plate, i.e., the edge of the crystal plate adjacent to the eyelet 14. When it is desired to form the mounting means from a single bifurcation, such bifurcation may be fabricated by bending and shaping the terminal to form a shoulder and bifurcation, by swaging the end of the terminal to form a shoulder and bifurcation, or by removing one of the pair of illustrated bifurcations from the illustrated terminals 12, 13. When the mounting means are constructed according to anyof the above teachings, at least one bifurcation or crystal plate mounting portion of each of the terminals will be disposed adjacent to a face of the crystal plate.

Preferably, the mounting means are not formed until after the eyelet and terminals have been assembled together to form a header. When a sealant is used to hermetically seal a pair of terminals in an eyelet, it is only necessary to place the terminals in the apertures of the eyelet, axially position the terminals relative to the eyelet, and maintain the relative positions of these elements until the sealant, whether vitreous or non-vitreous, becomes sufficiently rigid to maintain the terminals in assembled relation with the eyelet. With particular reference to the illustrated embodiment, it will be noted that since the terminals 12, 13 are not bifurcated prior to being secured to the eyelet 14, it is not necessary to orient the terminals with regard to the crystal plate mounting means during the above described steps. Any suitable means may be used for forming the bifurcations 22, 23 and such means includes but is not limited to apparatus such as a saw. By forming the mounting means after the eyelet and terminal is assembled, it will be appreciated that such means may be formed in exact alignment for receiving a crystal plate without stress-loading such plate. Furthermore, this procedure eliminates the necessity of orienting separate crystal plate mounting means that are assembled with the terminals. It should now be apparent that the process of forming crystal plate mounting means integral with the terminals after fabrication of a header is also useful even when such header comprises one or more terminals insert molded or otherwise secured to an eyelet made of a plastic or similar material.

When the terminals 12, 13 are supported in non-vitreous means, such as an organic adhesive material as illustrated, it is preferable that the terminals each be provided with stress isolating means for insulating the mounting means from stresses caused by bending, twisting, or otherwise stressing the free ends of the terminals, e.g., ends 12a and 13a. In the crystal unit 10, such stress isolating means include paddle sections 12b and 13b which are fonned by swaging the tenninals to provide debilitated segments between the ends thereof. In many applications, it nonnally would not be expected that a torque would be applied to the free ends 12a, 13a of the terminals. However, when a terminal is inadvertently bent to the dotted line position of tenninal 12 in the manner illustrated in FIG. 4, forces directed along a line generally perpendicular to the plane of the drawing and applied to the free end 12a can break the hermetic seal around the terminal 12 and stress-load the crystal plate 17. Actual tests have been made on two different header and crystal plate assemblies to illustrate the usefulness of the paddle sections 12b and 13b. One of these assemblies, herein referred to as assembly A, corresponded to the construction illustrated in FIG. 4 and the other assembly, herein referred to as assembly B, similarly corresponded except that the terminal corresponding to terminal 12 was not provided with any stress isolating means. After the terminals of assembly A were bent to the dotted line position of terminal 12 in FIG. 4, forces applied to the free ends of the terminals caused them to turn about an axis defined by the solid line position of terminal 12 in FIG. 4. Continued application of such forces caused the terminals to actually twist apart at the paddle sections, but at no time during the test was there any indication that the portion of the terminal between the paddle sections and mounting means moved relative to the sealant or that the hermetic seal was broken between such portions and the sealant, and at no time during the test was there any indication that a torque was transmitted to the mounting means. However, when a force was applied to the bent terminal 12 of assembly B, the body of the terminal started to turn in the sealant, the mounting means started to turn, and the crystal plate was mechanically stressed. When continued force was applied, the mounting means actually started to turn or rotate about an axis defined by the solid line position of terminal 12 in FIG. 4. As this occurred, the entire portion of the terminal embedded within the sealant started to turn relative to the sealant, thus breaking the hermetic seal therebetween, and the mounting means applied a stress to the crystal plate and actually fractured the comer of the crystal plate to which it was attached. It will be noted that the portions of the terminals 12, 13 embedded within the sealant 16 are substantially straight, i.e., the portions of the terminals above and below the debilitated sections are in line with each other. Therefore, the terminals 12, 13 may each be randomly oriented about the longitudinal axis thereof and have been positioned as shown in the drawings only for clarity of illustration. In summary, the terminals 12, 13 comprise straight or linear portions randomly oriented within the sealant 16, and the paddle sections 12b, 13b formed in such portions prevent the transmission of stresses from the free ends 12a, 13a of the terminals to the mounting means 22, 23. In addition, the sections 12b, 13b prevent the seal between the terminals and sealant 16 from being broken along the portions of the terminals between the mounting means and sections 12b, 1312 even when the sealant between the sealant and remaining portions of the terminals is destroyed.

When the terminals 12, 13 are stressed, the organic adhesive sealant 16 will not normally be cracked or chipped because it is relatively resilient. Preferably, the organic adhesive 16 is an epoxy material. Since epoxy will readily adhere to a wide range of materials that may be used to fabricate the eyelet 14 and terminals 12, 13, the specific coefficient of thermal expansion of such material is not as critical as it otherwise would be if the sealant were a relatively non-adhesive vitreous material. In the illustrated embodiment of the invention the eyelet 14 is made of aluminum and the terminals 12, 13 are made of tin coated copper and the sealant 16 is a conventional epoxy resin. When using this material the terminals 12, 13 are positioned in the apertures 26, 27 of the eyelet 14 as best indicated by FIGS. 3 and 4, and the uncured epoxy is dispensed into the eyelet 14 around the terminals. Then the eyelet, terminals, and sealant are heated to approximately 177 C. in order to cure the epoxy and form a solidified but relatively resilient sealant around the terminals 12, 13. The adhesive 16 adheres to the terminals and eyelet and maintains a hermetic seal therebetween; and, in order to reduce the strain placed on such seal during thermal cycling, the thermal coefficient of expansion of the epoxy 16 is modified so that it will approximately equal the thermal coefficient of expansion of the aluminum eyelet 14,i.e., about 25 X per degree Centigrade. The epoxy 16 readily adheres to the eyelet and terminals, and is relatively resilient and not readily cracked or crazed. Some of the other advantages attained by the use of the epoxy will be better understood and more readily explained by comparing some of the characteristics of a typical prior known vitreous sealant and the epoxy 16.

Accordingly, reference is now made to the graph of FIG. 5 which shows the relationship between the specific volume (cubic centimeters per gram) and temperature (degrees Centigrade) of these two materials. Curves C, D, E, and F illustrate the manner in which specific volume varies as a function of temperature of a vitrifiable glass sealant. On curve C, point G is representative of a typical melting point of the glass material and at temperatures above T the glass is liquid while at temperatures between T and T the glass is a supercooled liquid. The curves D, E, and F illustrate the relationship between specific volume and temperature of the glass after it has become vitreous, and whether the specific volume follows curve D, E, or F depends on the rate at which the glass is cooled. If the glass is cooled relatively fast, it will have a fictive temperature of T and a specific volume along curve D whereas slow cooling results in a fictive'temperature of T and a specific volume along curve F. The curve E and fictive temperature T correspond to one of an infinite number of cooling rates intermediate the two cooling rates corresponding to curves D and F. As will be understood, the temperature range T to T represents the transformation range" of the glass and vitrification of the supercooled liquid glass occurs at a temperature within this transformation range. The curves H and K approximately represent the relationships between specific volume and temperature of the epoxy 16, with the curve H illustrating the constant temperature volumetric change that occurs as the epoxy cures and the curve K illustrating the volumetric change of the cured epoxy as the temperature thereof is reduced to room temperature from a curing temperature T ofabout 177 C.

The slopes of the various curves shown in FIG. 5 are approximately indicative of the coefficients of thermal expansion of the glass and epoxy. With reference to the curves C, D, E, and F, it should be noted that when the molten glass is deposited in an eyelet, it will cool from the melting point T (from 900 to l,l00 C.) and exhibit a relatively constant coefficient of thermal expansion as a fictive temperature is approached. Then, depending upon the rate of cooling of the supercooled liquid glass, a fictive temperature between T and T will be reached and the glass will become vitreous. With further cooling the thermal coefficient of expansion will be reduced to from one-third to one-half of the coefficient of thermal expansion of the supercooled liquid glass.

It now should be apparent that when a vitreous material is used as a sealant, the eyelet and terminals must be subjected to a much higher temperature, i.e., T than is the case when an organic adhesive material is used which cures at a substantially lower temperature, i.e., T In addition, even though the rate of contraction of shrinkage (the slope of curves D, E, F) of a vitreous material may be predicted with a high degree of certainty, the actual percent change in specific volume, i.e., the actual shrinkage of the vitreous material, is relatively unpredictable unless very rigid control is maintained over cooling rates since the actual shrinkage of the vitreous material is dependent upon the rate of cooling and the fictive temperature at which the supercooled liquid glass becomes vitreous. Therefore, the actual strength of a hermetic seal that depends solely on the actual amount of shrinkage of a vitreous material is not precisely predictable. However, in the case of an epoxy, the actual shrinkage of the material after curing or solidification is relatively predictable and essentially independent of the cooling rate. In addition, the actual amount of shrinkage or volumetric change of the epoxy is not as critical as in the case of glass because the seal is aided by the adhesive qualities of the epoxy and does not depend solely on the compressive action of the sealant in the same manner as a vitreous glass seal. Other advantages of using an organic adhesive sealant should also now be readily apparent. For example, the tin coating on the copper terminals remains intact when the sealant I6 is used, whereas the tin coating would be quickly removed if it were exposed to molten glass.

The coefiicient of thermal expansion of the epoxy sealant 16 is approximately equal to that of aluminum, i.e., about 25 X 10 per degree Centigrade and the coefficient of thermal expansion of the vitreous glass typified by curves D-F is about 11 X 10 per degree Centigrade. This difference in coefficients is indicated by the different slopes of curves D-F and curve K, it of course being understood that the actual coefficient of thermal expansion of the vitreous material and epoxy 16 actually is a nonlinear function of temperature and that such coefficients have been treated as a linear function of temperature solely for the purpose of illustration in FIG. 5. However, the coefficient of thermal expansion may, for practical purposes, be considered to be fairly uniform over a relatively narrow temperature range and thus it will be appreciated that the relatively narrow range of processing temperatures required for an epoxy sealant provides the advantage that the total volumetric change of the epoxy is generally more predictable than the total volumetric change of a vitreous material that must be processed over a relatively wide temperature range. I

For the purposes of this application, the term epoxy material is meant to refer to the class of organic adhesive materials characterized by a molecular structure that includes a three member ring consisting of an oxygen atom attached to two adjacent carbon atoms and this term is meant to include catalysts, curing agents, and filler materials, whether organic or inorganic, that are used to extend or modify various properties of such material. In the exemplified construction, the epoxy resin was mixed with an aromatic polyamine based catalyst. The previously mentioned material that was used to modify the coefficient of thermal expansion comprised silica powder passable through a standard 325 mesh sieve and 40 parts by weight of such filler were added to 60 parts by weight of the resin-catalyst mixture.

The present invention alleviates the problems of processing a sealant at extremely high temperatures and the inherent difficulties that are encountered in such processing, including the necessity of exercising rigid control over temperatures and cooling rates of materials. In addition, the eyelet and terminals may now be fabricated from a wide selection of materials. As one example, the terminals may now be made of a malleable electrically conductive material, i.e., a material having the characteristics of copper or aluminum. The prior art techniques have placed limitations on the materials that could be used to fabricate eyelets because, among other things, of limitations imposed by the relatively low thermal coefficients of expansion of available vitreous sealants and the corrosive action of molten glass sealants. These limitatons have necessitated the use of relatively expensive alloy materials such as Kovar and other metal alloys. An extremely well suited inexpensive eyelet material is aluminum and even though some vitreous materials have been heretofore proclaimed as usable for coating aluminum, such vitreous materials have in fact had thermal coefiicients of expansion of only about 16.4 X 10 per degree Centigrade. Since the present disclosure teaches how to construct a crystal unit incorporating an aluminum eyelet and a sealant that can be made to very closely match the thermal expansion characteristics of aluminum, it will be appreciated that the present invention constitutes a substantial step forward in the art.

After completion of the steps of forming the bifurcations 22, 23 and securing the crystal plate 17 to the bifurcations, the flange 29 of the eyelet 14 is pressure or cold welded to the flange 31 of the aluminum envelope 11. During this step, high temperatures are avoided so that the crystal plate 17 will not be exposed to metallic or organic vapor contaminants that could mass-load the crystal plate. The stresses applied to the header and sealant 16 during this step do not damage the hermetic seal between the terminals 12, 13 and eyelet 14 because the sealant 16 is sufficiently resilient to withstand such stresses without crazing or cracking. The cold weld between the flanges 29 and 31 is accomplished by placing the flanges between a pair of dies and applying a sufficient amount of pressure to opposite sides of the overlapped flanges to cause the material in the flanges to flow and weld together. When this pressure is applied, the flanges 29, 31 are deformed and assume the cross-sectional configuration illustrated in FIGS. 3 and 4. Prior to cold welding the flanges 29, 31, the surfaces to be welded should be suitably cleaned, and during the cold welding process the cross-section or thickness of the overlapping flanges 29, 31 are preferably reduced approximately 70% along the weld line. Details of suitable cleaning and welding techniques are described in the previously identified Sowter patent and such description if specifically incorporated herein by reference.

The ease with which different sizes of crystal plates may be supported on a single size of header or on a pair of terminals spaced for proper registry with a printed circuit board will be best understood by having reference to FIGS. 6 and 7 wherein the envelopes of the illustrated crystal units have been omitted for purposes of clarity. In FIG. 6, the eyelet 32 and sealant 36 are substantially identical to the eyelet 14 and sealant 16 of the crystal unit 10. In addition, the crystal plate mounting ends 37a, 38a, of the terminals 37, 38 as well as the portions of those terminals embedded within the sealant 36 are substantially identical to the corresponding portions of the terminals 12, 13. However, the crystal plate 39 is physically smaller than the crystal plate 17. In order to accommodate this smaller crystal plate, the terminals 37, 38 have been formed with the bifurcated portions 40, 41 thereof directed toward each other so as to support the crystal plate 39 without mechanically stressing such crystal plate.

The embodiment illustrated in FIG. 7 differs from the embodiment of FIG. 6 only in that the bifurcated portions 44, 46 of the terminals 47, 48 are directed away from each other in order to support the crystal plate 49. As can be seen from a comparison of FIG. 7 and FIG. 4, the crystal plate 49 is physically larger then the crystal plate 17. In other respects, the embodiment of FIG. 7 is the same as the embodiment of FIG. 4 with the eyelet 51 being substantially identical to the eyelet 14 and the sealant 52 being substantially identical to the sealant 16. Although it would be possible to form the bifurcated portions of the terminals 37, 38 and 47, 48 prior to the time that such terminals are hermetically sealed in the eyelets, it is preferable to assemble non-bifurcated straight terminals with the eyelets without regard to the orientation of the terminals and then bifurcate the ends of the terminals and bend or otherwise form the crystal mounting plate ends of the terminals 37, 38, 47, 48 to accommodate the particular crystal plate that is to be mounted thereon. In the embodiments of FIGS. 6 and 7, as in the embodiment of FIG. 4, the bottom of the crystal plate is nestedly supported by the mounting means with at least one bifurcation of the mounting means disposed adjacent to a face of the crystal plate. In all three embodiments of the invention, bifurcations define the ends of a pair of terminals adjacent to a crystal plate within an envelope. As best shown in FIGS. 3 and 4, the peripheral bottom edge of the crystal plate is at least partially bounded by the ledge or shoulder at the base of the bifurcations, but the peripheral side edges of the crystal plate are not covered or enclosed by the mounting means. Each bifurcation on each terminal defines one side of a slot or crystal plate receiving portal that permits unrestrained lateral movement of the crystal plate. This structural arrangement facilitates the mounting of differently sized crystal plates on a given pair of mounting means and also ensures that the crystal plate will not be stress-loaded as a result of the mounting means engaging the peripheral side edges thereof.

From the foregoing description of the various exemplifications of the invention, it will be apparent that there is disclosed herein a new and improved piezoelectric crystal unit and method for making the same that overcome the aforementioned problems and disadvantages in the art and that accomplish the stated objects of the present invention.

While there has been illustrated and described herein what is at present considered to be preferred embodiments of the present invention and a preferred method of manufacturing a crystal unit, it will be appreciated that numerous changes and modifications are likely to occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A method of manufacturing a piezoelectric crystal unit comprising the steps of securing a pair of conductive malleable terminals to an eyelet, forming an axially extending mounting means in an end of each of said terminals after said terminals are secured to said eyelet, loosely supporting a crystal plate on the mounting means, mechanically securing the crystal plate to the mounting means, positioning an envelope around the crystal plate, and securing said envelope to the eyelet.

2. The method of claim 1 wherein the step of forming the mounting means in an end of each of the terminals includes the step of providing a slot in the end of each of said pair of terminals to define an axially extending bifurcation.

3. The method of claim 2 wherein the step of forming the mounting means in an end of each of the terminals further includes the step of positioning the mounting means relative to each other to accommodate the crystal plate to be mounted thereon.

4. The method of claim 1 wherein the step of securing the terminals to the eyelet comprises the step of forming a seal between the eyelet and terminals by dispensing an organic adhesive material into the eyelet adjacent the terminals and thereafter curing said organic adhesive material to form a resilient substantially solid mass.

5. The method of claim 1 wherein the step of securing the envelope to the eyelet includes the steps of positioning a flange on the eyelet and a flange on the envelope in overlapping relationship and cold welding the flange on the eyelet to the flange on the envelope.

6. A method of manufacturing a piezoelectric crystal unit comprising the steps of positioning at least one terminal in an aperture in a flanged metal eyelet of aluminum, dispensing an epoxy material into the eyelet adjacent to said terminal, curing the epoxy to secure the terminal to the eyelet, supporting a crystal plate on the said at least one terminal, positioning a flanged metal envelope of aluminum around the crystal plate, and cold welding the flange of said envelope to the flange of said eyelet. 

1. A method of manufacturing a piezoelectric crystal unit comprising the steps of securing a pair of conductive malleable terminals to an eyelet, forming an axially extending mounting means in an end of each of said terminals after said terminals are secured to said eyelet, loosely supporting a crystal plate on the mounting means, mechanically securing the crystal plate to the mounting means, positioning an envelope around the crystal plate, and securing said envelope to the eyelet.
 2. The method of claim 1 wherein the step of forming the mounting means in an end of each of the terminals includes the step of providing a slot in the end of each of said pair of terminals to define an axially extending bifurcation.
 3. The method of claim 2 wherein the step of forming the mounting means in an end of each of the terminals further incLudes the step of positioning the mounting means relative to each other to accommodate the crystal plate to be mounted thereon.
 4. The method of claim 1 wherein the step of securing the terminals to the eyelet comprises the step of forming a seal between the eyelet and terminals by dispensing an organic adhesive material into the eyelet adjacent the terminals and thereafter curing said organic adhesive material to form a resilient substantially solid mass.
 5. The method of claim 1 wherein the step of securing the envelope to the eyelet includes the steps of positioning a flange on the eyelet and a flange on the envelope in overlapping relationship and cold welding the flange on the eyelet to the flange on the envelope.
 6. A method of manufacturing a piezoelectric crystal unit comprising the steps of positioning at least one terminal in an aperture in a flanged metal eyelet of aluminum, dispensing an epoxy material into the eyelet adjacent to said terminal, curing the epoxy to secure the terminal to the eyelet, supporting a crystal plate on the said at least one terminal, positioning a flanged metal envelope of aluminum around the crystal plate, and cold welding the flange of said envelope to the flange of said eyelet. 